There are literally a thousand different uses for the slang version of "retarded" - (Quick note:) Usage-wise, it came before and set the precedence of the way we use "gay" in slang today - used correctly, they're practically interchangable.

"I thought it was a simple babysitting job, but I was wrong. Besides all the other stuff, I had to clean up after the retard, which was pretty retarded. To top it off, he set the carpet on fire, which I had to put out with the flame-retardent extinguisher. And after all of that, the parents only paid me $15. Babysitting is gay."

The Urban Dictionary T-Shirt

The Republian Political Party. Republicans are widely considered conservative and unwilling to make progressive social and financial policy changes for the betterment of society, thus RETARDING the advancement of society.

The Bush Administration and the Republican Congress have retarded progress in the United States over the past 10 years and caused a huge economic depression as a result of their retarded ways of thinking.